1. Field of Invention
This invention is in the field of clock pulse generation for a digital machine and methods therefore and, more particularly, is an apparatus for generating a substantially jitter free spectrum spread clock signal and method therefor.
2. Description of the Prior Art
The average amplitude of the spectrum of undesired radiation caused by clock pulses in a digital computer is reduced when the frequency of the clock pulses continuously deviates from a center frequency. The clock pulses with the continuously deviating frequency are generated by a spectrum spread clock.
Spectrum spread clock pulses are generated by providing a digital signal representation of an input angle to a circuit arrangement that includes a type of digital storage device known as an accumulator. The accumulator is connected to a reference clock pulse generator. The accumulator has an output that provides a digital signal representation of an accumulated angle which is incremented by the input angle in response to a reference clock pulse.
The output of the accumulator is connected to a programmable read only memory (PROM) at its input. The PROM provides a digital signal representation of the sine of the accumulated angle.
The output of the PROM is connected to a digital to analog converter (D/A) that drives a squaring circuit which provides input pulses to a phase locked loop. The input pulses are at a frequency directly related to the input angle. The phase locked loop provides the spectrum spread clock pulses.
Although the circuit arrangement described hereinbefore is suitable for generating the spectrum spread clock pulses, it is costly because of the number of circuit components included therein. In an alternative circuit arrangement of reduced cost, the PROM, the D/A, the low pass filter and the squaring circuit are eliminated. The phase locked loop is driven by the most significant bit (MSB) of the output of the accumulator.
It should be appreciated that when the MSB undergoes a transition from a logic "1" signal to a logic "0" signal or vice versa, the accumulated angle typically changes by more than an angle represented by a least significant bit (LSB) of the accumulator. As a result, there is usually jitter of the MSB that results in a corresponding jitter of the spectrum spread clock pulses.
The jitter of the MSB is reduced by increasing the frequency of the reference clock and decreasing the input angle. Heretofore, there has not been an economical way of generating jitter free spectrum spread clock pulses when the MSB is not jitter free.